Render using all cores except one (or N)
You will have to have a script file in each of your blends containing this:

import bpy
from multiprocessing import cpu_count

cores_idle = 1    # &lt;- SET THIS

# cpu_count() returns logical cores not physical
cores_available = cpu_count()
cores_enabled = max(1, cores_available - cores_idle)

for scene in bpy.data.scenes:
    scene.render.threads_mode = 'FIXED'
    scene.render.threads = cores_enabled


Give it a .py extension and check register so it is run by blender when you open the file (this should be also enabled in Preferences). You can also save this script into your Start Up file CtrlU.

Or for rendering you can execute it from the command line with --python, without having to modify your startup or .blend files.